Power combiners/dividers can be implemented in numerous configurations, and are typically specified for parameters such as maximum power dissipation, operating frequency range, insertion loss, VSWR, degree of isolation between input and output ports, and amplitude and phase balance. For purposes of brevity, the phrase power combiner/divider will simply be referred to herein as power combiner, as it is known that a power combiner can also be used as a power divider, depending on the direction of current flow.
A commonly used power combiner is the Wilkinson power combiner. The Wilkinson power combiner combines N loads or sources, while simultaneously providing isolation between those loads or sources. In general, the Wilkinson power combiner offers matched impedance conditions at all ports, a low insertion loss, and high isolation between output/input ports. Quarter-wavelength transmission lines are uniformly arranged to obtain isolation between ports by having reflected signals combine 180 degrees out of phase. Discrete isolation resistors are arranged in a network, where each resistor has one end connected to a common point and the other end connected to a different one of the transmission lines one-quarter wavelength from a common junction of that transmission line with the other transmission lines. The Wilkinson power combiner has bandwidth limitations that can only be improved by cascading multiple sections of Wilkinson combiners.
Power combiners can also be implemented with variable impedance transmission line. Here, a plurality of quarter-wavelength transmission lines are interconnected, so as to provide the input and output ports of the combiner. Each transmission line has a center conductor of constant cross-section (e.g., copper conductor), an outer conductor surrounding and coaxial with the center conductor and spaced radially therefrom, and a variable dielectric constant material between the center conductor and the outer conductor. This variable dielectric constant material transforms the transmission line impedance continuously from one end to the other to give very broad bandwidth. The center conductors of the transmission lines are connected together so as to provide input and output ports. A like plurality of resistors, each having a preselected resistance, are connected between the center conductors. Example such configurations are described in U.S. Pat. No. 5,796,317, titled “Variable Impedance Transmission Line and High-Power Broadband Reduced Size Power Divider/Combiner Employing Same”, which is herein incorporated by reference in its entirety. The resistors, however, have a capacitance to ground, which can cause the balanced currents to flow through the resistors, which in turn causes signal loss.
What is needed, therefore, is a power combiner that has very broad bandwidth and low signal loss.